By Cliff HarveyLow-carb diets have received criticism due to their perceived effects on thyroid function, and the belief that this will have a negative effect on general health, along with performance and fat-loss. But the real picture may be less than clear-cut.

A 16 week trial of a ketogenic diet for the treatment of type 2 diabetes also showed no significant effect on TSH levels [4], likewise no statistical difference was seen between groups in TSH levels in a randomised controlled trial of ketogenic diets for the treatment of obesity and hyperlipidaemia (including 119 participants over 24 weeks) [5]. No statistical difference was observed in T3 uptake between a very low carbohydrate diet group and control, with the VLCD group exhibiting marginally higher T4 levels [6].

In their paper on the effects of a eucaloric ketogenic diet LCHF pioneer Stephen Phinney and colleagues noted that despite significant reductions in T3 levels there were no concurrent effects of hypothyroidism noted, nor were there an associated reduction in oxygen uptake [1]. What does this mean?

This data suggests that there is:

An incomplete association between T3 levels and metabolic rate.

Not necessarily a detrimental effect of carbohydrate restriction on the regulation of thyroid hormone levels

Changes in T3 output are due to a potential lower need for T3 in the presence of lower carbs

This alters T4-T3 conversion rates

Conclusion

It appears that lower carbohydrate diets may cause there to be less requirement for T3, and the drop in T3 that is observed is not necessarily detrimental. This is an important distinction because if someone is still functioning optimally even in the presence of lower T3 levels there is more than likely not any particular health issue that needs to be addressed. Individual responses to LCHF and particularly very low-carb diets vary greatly. Anyone with a medical condition should seek sound advice from a qualified practitioner, knowledgeable in LCHF diets before embarking on a reduced carb nutrition plan, but it does appear that lower-carb diets may be appropriate, and are likely safe for thyroid function.